Great question! Broadly speaking, the major fires we see so often in the West today are products of the combination of 20th century fire suppression and encroaching development. We spent the better part of a century preventing wildfires while also intruding ever deeper into forests and other flammable landscapes with roads, houses, and recreational development, so when the inevitable human-caused fires ignited they burned hotter and more broadly than they might have in a natural landscape. That is not to say there were no fires prior to this development; indeed there is evidence of frequent natural and anthropogenic fire dating back centuries in the tree ring data as well as in historic records, and we of course know that Native Americans frequently used fire as a tool for both clearing land and driving game. But our modern approach to fire has stopped the natural cycle of fuel reduction and made what would have otherwise been small, localized fires larger and more destructive events.

The roots of this go back to the 1870s at least, when major fires in the cut-over lands of the pineries of MI/WI/MN took many lives and destroyed entire towns. (See the Peshtigo, Cloquet, and Hinckley fires as examples.) These were fires causes by the practice of piling up logging slash-- unwanted trees and waste --which were later ignited by lightening, sparks from trains, or other human-sources. With the rise of conservation in the late 19th century wildfire came to be seen both as a source of "waste" (i.e. timber that could not be used) and an opportunity for scientific control of nature-- fire suppression --while the economic and human impacts of these major post-logging fires made it politically expedient to steer public investments toward prevention.

Gifford Pinchot, the first Chief of the US Forest Service and the first formally-trained forester in the US, understood the role of natural and anthropogenic fire in forest health. He wrote an article for National Geographic in 1899 that noted "The forests which the first white explorers saw as they landed on this continent and gradually overran it were themselves the successors of others, which, through thousands of years, were burned down at intervals that we can no longer trace. There is but little of all the vast forest area of this country which does not bear, either in actual scars and charcoal or in the manner and composition of its growth, the marks of fire, and indeed it is more than probable that further investigation will greatly narrow the limits of those portions which may now seem to have been exempt." He went on to lament the "vast losses" caused by fire and to call for more study and resources directed to understanding wildfire, as an argument to grow the federal Bureau of Forestry (which he headed at the time).

In 1910, a few years after the US Forest Service was established, a series of massive fires burned across WA/ID/MT called "the Big Blowup" that further made the case for significant investment in fire suppression. About 4,000 Army troops were assigned to fight the initial fires at Pinchot's request, but ultimately over 3M acres burned and 85 people were killed. In the wake of the 1910 fires the US Forest Service went all-in on fire prevention, altering the natural fire ecology of the West in the process. While preventing/extinguishing anthropgenic fires (human-caused) perhaps made sense, the USFS also sought to prevent natural fires (most often lightening-caused) for most of the 20th century, thus leading to a massive buildup of fuel in western forests and ensuring that any fires that did ultimately start would be far worse than if they had occurred in a landscape that saw regular clearing by natural fire.

A turning point came in 1988 with the massive fires in Yellowstone National Park, which triggered an evacuation and nearly took the historic lodge. The cost of fighting that fire and the headlines it drew finally created the opportunity for fire ecologists and forest managers to be heard-- within a few years many federal agencies moved to a "let burn" policy for natural fires and began using controlled burns to reduce fuel loads so when fires did happen they weren't as destructive. But it will take decades to reduce the fuel buildup of a century of suppression, and meanwhile forest managers are dealing with major challenges like the massive insect-kills (pine bark beetle and spruce budworm) that have left thousands of acres of of dead trees standing around the west, waiting for a spark. Climate change isn't helping either-- in fact it's why some of these insects are now able to winter over and kill trees --while droughts and fire have obvious links.

The unquestioned authority of US wildfire history is Stephen Pyne, who literally wrote the book on fire. Many books in fact. His short overview America's Fires: A Historical Context for Policy and Practice (Forest History Society: 2010) provides a clear analysis of the evolution of fire policy in the 20th century and the impact of fire suppression on Western landscapes. His Year of the Fires: The Story of the Great Fires of 1910 is a fascinating historical read, while many of his other titles straddle the line between history, policy, and fire science in a very engaging way.

There is a nice listing of important major wildfires on the Forest History Society web site that will provide a sense of scale and human cost to some past fires. Steve Pyne's books will give you more info than you'd ever need if you'd like to learn more.